A big enough rise of global temperatures would eventually melt the
world's glaciers, and indeed a retreat of mountain glaciers since the
19th century was apparent in many regions. That would release enough water
to raise the sea level a bit. Worse, beginning in the 1960s, several glacier
experts warned that part of the Antarctic ice sheet seemed unstable. If
the huge mass slid into the ocean, the sea-level rise would wreak great
harm, perhaps within the next century or two. While that seemed unlikely
(although not impossible), by the 1980s scientists realized that global
warming would probably raise sea level enough to affect populous coastal regions. Around 2000, changes in Greenland and Antarctica raised worries that the rise might become dangerously rapid. The problem would be redoubled if stronger storms sent water surging inland.

Glaciologists, the scientists who study how ice behaves in
seriously large quantities, have a special interest in floods. They
even have their own word, jökulhlaup (from Icelandic), to describe
the spectacular outbursts when water builds up behind a glacier and
then breaks loose. An example was the 1922 jökulhlaup in Iceland.
Some seven cubic kilometers of water, melted by a volcano under a
glacier, had rushed out in a few days. Still grander, almost unimaginably
grand, were floods that had swept across Washington state toward the
end of the last ice age when a vast lake dammed behind a glacier broke
loose. In the 1940s, after decades of arguing, geologists admitted
that high ridges in the "scablands" were the equivalent of the
little ripples one sees in mud on a streambed, magnified ten thousand
times. By the 1950s, glaciologists were accustomed to thinking about
catastrophic regional floods.

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Also within their purview was flooding on
a far grander, but far slower, scale. Since the heroic polar explorations
of the late 19th century the world had known that great volumes of
water are locked up in ice sheets. If there were substantial melting
of the Greenland ice cap, and especially of the titanic volume of
ice that buries Antarctica, the water released would raise the oceans
in a tide that crept higher and higher for millennia. It had happened
before  geologists identified beaches far above the present
sea level, cut by waves in warmer periods when the Earth was entirely
free of ice. In the last interglacial period, some 125,000 years ago,
the planet had reached a temperature about as high as was likely to
come from greenhouse warming in the next century or two. Back then,
even though most of Antarctica had remained ice-covered, the sea level
had been at least seven meters (more than 20 feet) higher than at present. This
was about what would be expected if most of Greenland melted. The
next time that happened, sea water would swamp coastal regions where
a good fraction of the world's population now lived. All this became
familiar to anyone who followed scientific discussions of global warming.

Up to the 1960s, scientists expected that global warming caused by greenhouse gases, if it happened at all, would steal in gradually over many centuries. So the threat of flooding lay in a comfortably
vague and remote future. To be sure, a few scientists had begun to
imagine more abrupt change if the melting of the ice itself brought
on conditions that accelerated the warming. Transitions between glacial
and warm climates  and back again  might come in a matter
of only a few centuries if not faster. As one example, in 1947 the New York Times
quoted a prominent Swedish geophysicist, Hans Ahlmann, who suggested
that a global warming might be underway that could eventually bring
a "catastrophic" rise of sea level as glaciers melted. "Peoples living
in lowlands along the shores would be inundated," he explained, calling
on international agencies to undertake studies as an urgent task.
Most scientists, however, expected that within the foreseeable future,
the main effect of any global warming on ice would be to shrink the
ice pack on the Arctic Ocean. Since that ice was floating, it could
melt entirely away without changing sea level at all, just as the level of water in a glass does not change when a floating ice cube melts.(1)

Glaciers on land could affect sea level, and they were notoriously
sensitive to climate. Advances and retreats of glaciers in the Alps
in particular had been conspicuous for generations, reacting to small
changes not just in temperature but also in the amount of snowfall.(2*) In 1962, John Hollin opened
up speculation about how relatively small climate changes might also
affect ice in Antarctica. He argued that great volumes of ice there,
piled up kilometers high and pushing slowly toward the ocean, were
held in place by their fringes. These edge sheets were pinned at the
marginal "grounding line" where they rested on the ocean floor. A
rise of sea level could float an ice sheet up off the floor, releasing
the entire stupendous mass behind it to flow more rapidly into
the sea.(3*)

The idea was picked
up by Alex Wilson, who pointed to the spectacle of a "surge." Glaciologists
had long been fascinated by the way a mountain glacier might suddenly
give up its usual slow creeping, to race forward at a rate of hundreds
of meters a day. They figured this happened when the pressure at the
bottom melted ice so that water lubricated the flow. As the ice began
to move, friction melted more water and the flow accelerated. Could
the ice in Antarctica become unstable in this fashion? If so, the
consequences sketched by Wilson would be appalling. As the ice surged
into the sea, the world's sea-coasts would flood. And that would not
be the worst of humanity's problems. Immense sheets of ice would float
across the southern oceans, cooling the world by reflecting sunlight.
It could bring a new ice age.(4*)
Hollin joined in by publishing observations of deposits in England that recorded
past sea levels, showing rapid rises of as much as ten meters. It
could happen any time, he thought, perhaps in mere decades 
or even faster if the sea-level change set off tsunamis. He pointed
to unusual features that suggested an abrupt disaster, such as "the
curiously intact remains of large mammals" buried whole.(5)
Few scientists gave much credence to any of these speculations. The
ice that covered most of Antarctica, in places more than four kilometers
thick, seemed firmly grounded on the continent's bedrock.

However, in 1968 John
Mercer, a bold and eccentric glaciologist at Ohio State University,
pointed out a problem: the West Antarctic Ice Sheet (WAIS). This is
a smaller — but still enormous — mass of ice, separated by a mountain
range from the bulk of the continent. Adventurous traverses of the ice during the International Geophysical Year 1957-58 had shown that much of the base of this mass was below sea level. Mercer argued that it
was held back from flowing into the ocean, in a delicate balance, only by the shelves of
ice floating at its rim. These shelves might disintegrate under a
slight warming. The much larger mass of ice corked up by the shelves
would then be released to slide into the ocean and disintegrate into icebergs. Just so, Mercer suggested,
a collapse of ice sheets into the Arctic Ocean might have caused the more local,
but remarkably sudden, cooling of the North Atlantic around 11,000
years ago that other scientists had identified. A West Antarctic Ice
Sheet collapse could be very rapid, Mercer said. The sea level would
not rise as far as it would rise if all of Antarctica surged, but it would be bad
enough  up to five meters, he estimated (16 feet; calculations
decades later pinned down the number at around 11 feet). Much of the
world's population lives near the shore. Such a rise would displace
more than a billion people and force the abandonment of many great
cities. Mercer thought it could happen within the next 40
years.(6*)

Mercer published his worries in an obscure conference report, and although he
wrote forcefully, he did not push his views on colleagues in the
personal encounters that are crucial in a small community of specialists
(he much preferred to be out doing fieldwork, often in the nude).
The few specialists who heard of his ideas were not impressed. The
problem, one of them complained, "could be argued indefinitely if
it is not quantized."(7)

In fact glaciologists had been working for decades on ways to calculate
numbers for the flow of ice masses. In the 1970s they made rapid
progress in formulating abstract mathematical models and putting
the powerful new computers to work. The calculations, with many
approximations, suggested that the West Antarctic Ice Sheet was
indeed unstable. Apparently the floating ice shelf that held it
back could break up with surprising ease, and the whole mass might
begin sliding forward.
One scientist who made a landmark calculation, Johannes Weertman,
concluded that it was "entirely possible" that the West Antarctic
Ice Sheet was already now starting its surge.(8)

The scattering of climate specialists and geologists
who paid attention to ice sheets viewed the models as highly speculative.
It seemed scarcely possible that anything as massive as the the West
Antarctic Ice Sheet could disintegrate in less than a few centuries.
But if you took a long enough view to be concerned about the next
few centuries, a surge that dumped a fifth of a continent of ice into
the oceans would be no small thing, and they could not rule it out.
The picture fitted with a new feeling that was emerging in the climate
community, a feeling that the climate system in general was unstable
or even radically chaotic.

Concern sharpened in
1975 when Cesare Emiliani at the University of Miami reported measuring
deep-sea cores that showed a shockingly rapid rise of sea level 
a rate of meters per decade  around 11,600 years ago. (He remarked
that this was exactly the time Plato had given for the fall of Atlantis!)
Emiliani thought the cause of the flooding might not have been an
Antarctic surge, but water rapidly released from enormous lakes that
had been penned up behind the North American ice sheet, a titanic
jökulhlaup. In places like Florida where the land sloped gently
into the ocean, he wrote, "the sea would have been seen to advance
inland 300 feet in... a single summer."(9) Other areas at risk included the Nile
Delta and the Netherlands. Science journalists made sure that the
more spectacular warnings reached a broad public.(10)

Meanwhile radar surveys from airplanes showed that the ice of West
Antarctica moved toward the sea not as a single sheet but through
a set of enormous ice streams. Terence J. Hughes (who started out
studying metallurgy but moved on to a different sort of solid material)
and other glaciologists developed increasingly elaborate models of
ice sheet dynamics.(11) They
showed how a slight shift in conditions could prompt an ice shelf
to break up into flotillas of icebergs. Looking over the new data
and theories, Mercer worried that most climate experts still assumed
that ice sheet changes would take many centuries. In 1978, he finally
caught their attention with an article in the leading journal Nature,
contending that because of global warming from humanity’s use
of fossil fuels, "a major disaster... may be imminent or in progress."
Mercer admitted that the computer models were loaded with uncertainties,
but "there is, at present, no way of knowing whether they err on the
optimistic or the pessimistic side."(12)

Mercer, Hollin and Hughes had a chance to argue their case to a
group of experts at a meeting convened in April 1979 in Annapolis,
Maryland. One participant noted in his diary that their arguments
convinced him that the deglaciation of West Antarctica was "a plausible
hypothesis." The majority felt that this was "not a cause for immediate
alarm however. We are talking about centuries."(13)
In a published review, a trio of experts laid out arguments explaining
why the collapse of an ice sheet would probably take several centuries
to run its course. Yet they admitted that "little is known about the
glaciers," and a 5-meter rise in sea level could possibly happen within
a century. "Mercer's warning," they concluded, "cannot be dismissed
lightly."(14)

That continued to be the most common view through the 1980s.
Studies found that an ice sheet collapse was likely to take centuries
rather than decades, but experts knew too little about the behavior
of Antarctica's mammoth ice rivers to agree on any firm conclusion.
Field glaciologists, a small but hardy group, measured one or another
ice sheet as best they could at a few scattered locations. They found
ice streams moving consistently at speeds of hundreds of meters a year, far faster
than ordinary mountain glaciers. Meanwhile, their mathematically-minded
colleagues back home constructed simplified models for the flow.(15) Some studies foresaw the possibility of a sea-level rise
of two or three meters (6-10 feet) by 2100, but most found this unlikely
so soon. In particular, for a 1983 National Academy of Sciences report,
the dean of oceanographers, Roger Revelle, estimated that within the
next hundred years the sea level would probably rise some 70 cm (about
two feet). That would be harmful but not catastrophic. He did worry,
however, about an Antarctic collapse later on.(16*)

Some rise of sea level in the coming century seemed not just possible,
but nearly certain. The oceans had already risen 10 or 20 centimeters
in the 20th century, about ten times as fast as the average sea-level
rise in previous millennia. Just where all the water had come from
remained uncertain. As one example, it was not until the 1990s that
experts realized that significant volumes of water were engaged by
human activities like irrigation and building reservoirs, and they
could not say whether the net result of such activities was to take
water from the oceans or to put more in.(17)

One contribution to the sea-level rise was entirely clear. Water
expands when heated. The consequences may seem obvious, but amid all
the talk of melting glaciers, for decades nobody seems to have given
a thought to other simple effects. Finally in 1982 two groups separately
calculated that the global warming observed since the mid-19th century
must have raised the sea level significantly by plain thermal expansion
of the upper ocean layers. But a thermal expansion could not account
for all of the observed rise. The scientists figured the rest came
from melting glaciers (most of the world's small mountain glaciers
were in fact shrinking).(18)

The rising waters might help the West Antarctic Ice Sheet float
off its moorings and slowly break up. Even if that never happened,
there would still be problems. Scientists warned that tides would
probably mount a half meter or even a meter and a half higher by the
end of the next century, bringing severe harm to coastal regions.
Beaches would erode back hundreds of feet. Salt water would advance
into fragile estuaries. Entire populations would flee from storm surges.(19*)

While the calculations of thermal expansion were straightforward,
the actual sea-level rise would depend on a much tougher problem 
what would happen to the ice sheets of Greenland and Antarctica? So
long as they did not surge and disintegrate, global warming would
not necessarily make them dwindle. A warmer atmosphere would hold
and transport more water vapor, so it would drop more snow. Thus the
polar ice sheets might actually grow thicker, withdrawing water from
the oceans. The future sea level depended crucially on just what happened
to glaciers and ice sheets, one pair of experts concluded, and predicting
that would be "a daunting task."(20)

To sketch out an answer to the great question of ice-sheet collapse,
since the early 1980s increasing numbers of scientists had bundled up in parkas and gone
out onto the windswept wastes of Antarctica. It was grueling work,
isolated and dangerous. Researchers measuring the ice streams
learned to travel with their ski-mobiles roped together like mountaineers (despite precautions, in 2016 glaciologist Gordon Hamilton died when his snowmobile plunged into a hidden crevasse). Their difficult
goal was to measure the motions of the immense slow ice currents,
using radar pulses, seismic measurements, and boreholes to study how
ice moved over the rock beneath. One example was a scientist who had
been skeptical of surge models  he recalled that he "felt the
whole thing was like a house of cards"  but who changed his
mind when he discovered that a kilometer-thick Antarctic ice stream
rested not on bedrock but on a layer of slippery mud.Another unsettling
discovery was that in recent centuries some of the great ice streams
had stopped or started moving, for no clear reason.(21)

Far more such data would be needed to bring a definitive answer. The
dynamics of ice sheets and the streams that fed them turned out to
be, like most things geophysical, a complicated snarl of influences.
Experts could not even agree on whether the West Antarctic Ice Sheet
had disintegrated during previous warm epochs over the past few million
years. The past sea-level rises might have come from Greenland ice,
or from something else entirely. But according to evidence developed
in the 1990s, during a dramatic episode at the end of the last ice
age, something had once raised the sea level 16 meters within
three centuries. The rate of rise might have reached two feet per
decade. The West Antarctic Ice Sheet was the most likely source of all that water. The
rush of new data fed what one observer called "polite but emotional
debate" among experts. And there were indeed WAIS experts now. Since the 1980s a little interdisciplinary, international community had been taking shape in ad hoc workshops at various locations.(22*)

Meanwhile a powerful new tool, satellite images, revealed that
some of the smaller floating ice shelves poking out from the peninsula
that projects from the Antarctic continent were rapidly disintegrating.(23)
It was not clear whether the changes had anything to say about the
possibility of a catastrophic ice-sheet collapse. In these little-known
regions, the changes might have been a type of normal, regional event,
which just had not been noticed before the age of intensive global
monitoring. Yet the public's concern about global warming was reinforced
from time to time when satellite images showed tabular icebergs bigger
than cities floating off. And scientists began to doubt this was normal.
After all, back in 1978 Mercer had called for keeping an eye on just
these ice shelves, contending that their breakup would be "one of
the warning signs that a dangerous warming trend is under way in Antarctica."
He had predicted still more specifically that the collapse of ice
shelves would start at the northern end of the Antarctic Peninsula
and proceed south, and indeed by 1996 the five most northern ice shelves
were shrinking rapidly, but not the more southerly ones.(24)

In the 1980s and '90s specialists in glacier flow worked up increasingly
complex ice-sheet models, using the rapidly expanding power of computers to incorporate essential features such as heat flow within the ice.(24a) Entirely aside from the question of Antarctic
surging, these models might be useful in explaining the ice ages.
It seemed increasingly likely that the reason ice sheets came and
went in cycles of around 100,000 years had something to do with the
length of time needed for a continent of ice to form and flow and
melt, while the entire rocky crust beneath it sluggishly sank or rebounded
as the weight of ice grew or diminished. Nothing else on Earth seemed
to change on the right timescale.

The models failed to answer the question of how fast a major ice
sheet could surge into the ocean. The improved models did show, reassuringly,
that there was no plausible way for a large mass of Antarctic ice
to collapse altogether during the 21st century. According to these
models, if the West Antarctic Ice Sheet diminished at all, it would
discharge its contents only slowly over several centuries, not placing
too heavy a burden on human society. Yet scientists could not altogether
rule out the possibility of a shocking surprise in some future generation.
The West Antarctic Ice Sheet remained what one expert had called it
a quarter-century earlier  "glaciology's grand unsolved problem."(25)

Scientists were still less able
to answer the question of whether climate change was gradually melting
the rest of the world's glaciers and ice caps, or instead was adding
snow to them. In "those huge areas where little or no information
is available," an expert explained in 1993, "almost anything might
be happening." But in 2005 a survey of mountain glaciers around the
world found that most of those for which historical records existed
had been shrinking since 1900. Some that had survived for many thousands
of years were vanishing, a striking sign of unprecedented climate
change. Experts could only speculate how far this might affect sea
level. Would it be counteracted by the increased snowfall
that some models predicted global warming would bring in the remote
dry highlands of Antarctica?(26*)

As scientists turned
increasing attention to ice movements, they discovered many kinds
of changes, thanks to satellites and airplane overflights
as well as increasingly precise measurements by grueling expeditions
on the ice itself. "Perhaps the most important finding of the past
20 years," a glaciologist reported in 2002, "has been the rapidity
with which substantial changes can occur on polar ice sheets." Warmer
ocean waters were melting the underside of ice sheets by tens of
meters a year, altering where grounding lines pinned them. Entire
floating ice shelves, some of which had been in place for thousands
of years, were rapidly thinning, or astonishing experts by breaking
up completely. Ice streams that had been held behind the disintegrating
shelves accelerated.

Most scientists had figured that even after the air got warm enough
to melt the surface of an ice shelf, it would take millennia for the
entire great mass to melt away. It turned out, however, that meltwater
could seep down into crevasses, refreeze there and wedge them wider,
prying apart a thick sheet in months. Meanwhile the gradually warming
seawater worked to break up the ice from beneath. None of this had
been foreseen by the crude computer models of ice behavior.

Modelers scrambled to incorporate the new
concepts. Revised computer simulations and further observations
confirmed the idea, originally so speculative, that removing an
ice shelf could dramatically speed up the drainage of glaciers "corked
up" behind it. In 2004 evidence was published that some of
the enormous ice streams leading from the West Antarctic Ice Sheet
to the ocean were also speeding up. Scientists were no longer sure
how many centuries it might take to drain the entire sheet. "The
response time scale of ice dynamics is a lot shorter than we used
to think it was," admitted a leader of the research.(27*)

Only a few people were trying to make computer models of any of
these processes, and their models remained primitive. A mass of ice
is an odd substance, something between a fluid and a solid, no easy
thing to simulate. And far too little data had been gathered from
the surfaces of these perilous and remote wastes, let alone from their
buried base. The most modelers could say was that "The latest theoretical
advances have done nothing to allay fears concerning the potential
instability of marine ice sheets."(28)

Meanwhile, starting around 2000, a few studies raised
the additional possibility that the Greenland Ice Sheet, contrary
to what most scientists had figured, might not be comfortably stable
over the next few centuries. In the warmer summers the snow on the
surface would get wet, and become darker. So it would absorb more
sunlight and warm still more. Under one speculative scenario, rivers
of water would drain through deep holes ("moulins") straight
to the bottom of the ice and lubricate it. That might provide, as
one team put it, "a mechanism for rapid, large-scale, dynamic
responses of ice sheets to climate warming." Another mechanism might be thinning and crevassing at an ice stream's front end, due to warmer ocean water, causing a speedup that propagated upstream. As the flow of its
huge ice streams accelerated, the Greenland ice cap would thin around
the edges. As the ice surface sank to lower altitudes where the air
was warmer, it could melt all the faster. Conceivably, an armada of
icebergs would invade the North Atlantic and melt, as had happened
around the end of the last ice age. At that time the sea level had
risen at a rate that would be catastrophic for coastal areas. The
process would presumably take centuries to run its course, or more
likely millennia, but glaciologists could only speculate about the
probability and timing of such a misfortune.(29*)

A 2006
analysis of satellite radar data found that the velocities of large
ice streams in southern Greenland had doubled in the past five years
— something most experts had thought was impossible. Perhaps
the speculations about lubrication of the base of an ice stream were
correct? The Greenland ice streams soon slowed down again, however,
showing that the lubrication was temporary; a long-range study reported
that these particular streams were discharging ice into the sea no
faster, on average, than a decade earlier. Glaciologists were not
reassured. Considering how ice streams around Antarctica had also
been observed to accelerate and slow down suddenly, it seemed that
these systems were more sensitive to perturbations than the scientific
community at large assumed. Moreover, a new satellite was transmitting
disturbing data. It measured gravitational force so sensitively that
it could detect changes in the mass of an ice sheet from year to year.
Both Greenland and West Antarctica were in fact losing substantial
amounts of ice into the oceans. Observers were dismayed to see mass around the margins of Greenland dwindling at a rate that doubled in less than a decade.

What would be the end-point? Some 400,000 years ago the temperature had been only a few degrees higher than at present, but the sea level had been more than six meters higher. An ingenious study of sediments found that southern Greenland had been largely ice-free at that time, and thus the island had been a main contributor to the sea-level rise. By the end of the century, if emissions of greenhouse gases continued, our civilization might well have locked in a grand change of the ice sheet and therefore of sea level.(30a*)

The losses from Antarctica were still more surprising. Since the 1980s, European and American expeditions had been measuring the ice streams held back behind the Ross Ice Shelf and adjacent regions; they had found no disturbing acceleration. But there was a second region, Pine Island Bay, where narrower ice streams fed into the ocean. This was one of the most inaccessible places on the planet, with terrible weather besides, and it had scarcely been observed even from the air. Yet the terrain was such that already back in 1981, Terry Hughes had suggested that warming might accelerate the ice streams. He had called Pine Island the "weak underbelly" of West Antarctica.(30b)

The region was lit up as by a flash of lightning with the 1991 launching of the satellite ERS-1, "the single most effective tool ever devised for measuring glacial change." The satellite's radar, peering through clouds and the long Antarctic night, could measure the ice surface with amazing precision. In 1998 Eric Rignot reported that the line where the Pine Island glacier was held back by the seabed had retreated five kilometers within half a decade. He assumed that the glacier's floating tongue was being eroded underneath by the warmer ocean waters. Another group found that the entire glacier basin feeding into the ice stream was losing altitude.(30c)

Eric Rignotcourtesy NASA

Glaciologists rushed to study the neglected region. By 2009 they found that the ice streams entering Pine Island Bay were rapidly accelerating, and the basin that fed them was dropping 16 meters a year. "We don’t know really know what's going to happen to the ice," remarked a British team leader. Eventually the ice streams might surge much as Hughes had warned. However, the whole process was expected to act at a truly glacial pace. Most experts felt that at worst West Antarctica might contribute ten centimeters or so to the total sea-level rise of the 21st century. Bigger problems would come, gradually but inexorably, in the 22nd century and after. Observational and computer studies published in 2014 confirmed this as a near certainty: there was nothing in the seabed around Pine Island Bay to pin down the Pine Island glacier, the even bigger Thwaites Glacier, and others and keep them from retreating over the next half-dozen centuries. They would spill gigatons of West Antarctic ice into the ocean. The collapse, Rignot announced to widespread press attention, had become "irreversible... it has passed the point of no return." In recent years scientists had been warning in general that the climate system could have "tipping points" (more formally, "critical thresholds"). We had now passed one of these at least.

Some additional light was shed in 2013 by a successful effort, by a consortium of more than a hundred scientists, to extract ice from Greenland as far back as the Eemian period. In that period (about 120,000 years ago) the world had been nearly as warm as it was likely to get in the 22nd century — and the sea level had been roughly 20 feet higher. It turned out that Greenland had not all melted away at that time. Therefore a majority of the sea-level rise must have come from Antarctica. The news added to scientists' worries. Computer models of ice sheets were unreliable and it was a mystery how fast the Antarctic ice might collapse.(30d*)

Scientists had thought the much larger volume of ice in East Antarctica would be stable due to its extreme cold. But studies in the early 2010s suggested that this sheet too was vulnerable as warming sea water undercut the glaciers that controlled the outflow. Expeditions probing the topography underlying East Antarctica found that, as with West Antarctica, there were regions with massive amounts of ice that were unstable and could collapse into the oceans. It was a problem for future centuries, however, not the present one.(31)

More immediate worries were raised by news that the Arctic Ocean ice pack was shrinking far
more rapidly than any model had predicted. The ice was dwindling to an unprecedented extent not only in area but, still more, in thickness and thus in total volume. By 2007 scientists were predicting
that a "Northwest Passage" across northern Canada would
be ice-free in summer decades earlier than expected — and already in 2016 a luxury cruise ship did make the trip. Of course, the disappearance of floating ice would not raise the sea level, but it could change weather patterns around the Northern Hemisphere. And the unexpected shrinking confirmed not only that
the ice component of the climate system was poorly understood, but
that our ignorance was concealing mechanisms that made for rapid changes(31a).

On the other hand, people who wished to deny that global warming was a problem pointed to the sea ice around Antarctica. In the few decades it had been monitored its area had expanded. To be sure, the increase was irregular and of slight magnitude, only the sum of increases in some regions and decreases in other regions — nothing like the spectacular dwindling of both the extent and the thickness of the Arctic ice pack. A variety of complex factors influenced the formation of Antarctic sea ice. Alongside normal cyclical changes there were global-warming-related changes in wind patterns and ocean currents and even the salinity of the seawater (as the melting glaciers of Antarctica dumped ever-larger volumes of freshwater into the sea). Experts could only agree that the subject was complicated. Indeed, a leading computer modeling group had predicted as far back as 1991 that during the first decades of global warming, the Antarctic Ocean would not warm up in the manner of the Arctic Ocean. The claim that the Antarctic sea ice fluctuations argued against global warming collapsed in 2017 when the area shrank to a record low.(31b)

One thing was certain. If temperatures
climbed a few degrees, as climate scientists now considered likely,
the sea level would rise simply because water expands when heated.
This is almost the only thing about global change that can be calculated
directly from basic physics. The additional effects of glacier and
ice sheet melting remained highly uncertain (scientists were still
arguing over how much of the 20th century’s sea-level rise
was due to heat expansion and how much to ice melting). One hint gradually became apparent, and was pinned down after 2000 by satellite measurements: the rate of sea-level rise was accelerating. Since the 1980s the sea level had been rising much faster than at any point since records began in the 1880s. Indeed wide-ranging geological surveys showed that the rise over the past century was beyond anything seen for thousands of years, probably tens of thousands of years. Shooting up on graphs like the blade of a hockey stick, the sea-level rise was increasingly recognized as responsible for a large part of the coastal flooding that had begun to endanger communities on the East Coast of the United States.(31c)

When the authoritative
Intergovernmental Panel on Climate Change (IPCC) issued its 2007 report,
the authors found the new ideas about ice sheets altogether uncertain. So
for their sea-level predictions they stuck with the known rise from warming plus the old, classic models
for ice processes. They took no account of the possibilty of surges (the
report was based on data published through about 2005, which left
out some of the most disturbing reports). Back in 2001 the IPCC had offered a rough guess for the total rise expected by the end of the 21st century — perhaps half a meter, give or take a bit — and the authors of the 2007 report came up with much the same numbers.
This refusal to include the possibility of ice-sheet collapse brought sharp criticism from some experts and others who worried about catastrophic sea-level rise.(32)

Even before the new results came in from Greenland and Antarctica, some scientists had been worrying that the rise might be double the 2007 IPCC report's upper limit. Now they were still less willing to rule out the possibility of a rise of one meter, or even two. Such rapid rates, it turned out, had been experienced in past geological ages similar to the present.(32a) By 2012 many experts were projecting a rise of a meter if not more. Backing this up was a comprehensive international study that found Antarctica and, still more, Greenland were losing mass at rapidly accelerating rates. In August 2012 the entire surface of Greenland was seen to be melting, with pools everywhere (and the pools, darker than the snow surface, would absorb still more sunlight). As one expert ruefully remarked, "The motto for early 21st Century cryospheric science might be, 'that happened faster than I thought it would'."(32b)

Meanwhile some senior glaciologists and others argued that it was a mistake to concentrate on what seemed most probable. What about processes that, although perhaps not likely, would be catastrophic if they did come to pass? Ignoring an unknown did not make it go away.

In its next major report, issued in 2013, the IPCC responded to the criticism. Now it projected a sea-level rise anywhere from 0.3 meter (if the world promptly launched vigorous emission reductions) to one meter. The latter was a conservative limit for what was "likely," and the panel warned that "there is currently insufficient evidence to evaluate the probability of specific levels above the assessed likely range." But they conceded a possibility of several tenths of a meter more if the WAIS started to collapse, that is, up to a 1.5 meter rise by 2100. (Things would get progressively worse in the following centuries, but few people concerned themselves with that.)

Some experts thought even the IPCC's 1.5 meter rise by 2100 might be an underestimate of what was possible. Computer models of ice-sheet collapse remained speculative; according to one model, the WAIS could collapse in a matter of decades. A prestigious group of scientists warned in 2015 that it was possible for the seas to rise several meters in the space of half a century. That showed, as a journalist noted, "just how far from resolved, scientifically speaking, the question of danger levels remains."(32c)

One meter of sea-level rise might not sound
like much, but in many areas it would bring the sea inland a hundred
meters or more (a few hundred feet), and even farther if storm-driven
surges grew stronger. While such a rise would not be a world disaster,
in the late decades of this century it would bring significant everyday
problems, and occasional storm-surge catastrophes, to populous coastal
areas from Bangladesh to New Orleans.

Scientists had warned
for decades that New Orleans was at risk from hurricanes, and some had pointed out
that the chance of disaster would mount as global warming raised the sea level
and perhaps increased storminess. But after the catastrophe in August 2005 some experts
asked whether Hurricane Katrina would have devastated the city, if
the heat in the Gulf of Mexico's waters — a main source of the
storm's energy — had not been higher than normal? Such a question
can never be answered for a single event. The important question is
not what global warming did in one case, but what it would mean for
the future probability of terrible hurricanes and typhoons.

Scientists had only a sketchy idea of how tropical storms worked.
Nevertheless, when the 21st century began, nearly all experts had
been confident that tropical storms would not become seriously worse
for many decades. Even Kerry Emanuel, who had explained in 1987
how a warmer sea surface would provide energy for greater storms,
had not expected a noticeable change anytime soon. But when he analyzed
decades of data on tropical storms, he found a disturbing trend.
While the number of hurricanes and typhoons had not been
increasing, the intensity of the worst storms seemed to
have climbed in recent decades. The rapid increase in destructive
power, so different from what experts had expected, correlated surprisingly
well with the observed rise of sea-surface temperatures. "For
the first time in my professional career," Emanuel recalled,
"I got alarmed." In mid 2005 he published a warning of gathering
danger. It attracted widespread attention three weeks later, when Katrina
struck.(33)

Meanwhile a separate group had gotten similar
results. But other meteorologists stuck by their earlier conclusions.
A fervent, sometimes personal controversy broke out. The experts of
the old school insisted that the record of tropical storm intensities
was only guesswork for most of the 20th century, especially in the
vast, unvisited spaces of the Pacific. If there had indeed been a
change in hurricanes, they supposed it was only a phase in a normal
North Atlantic cycle. Computer models varied, some projecting little
change, others predicting a modest increase in the intensity of great tropical storms by the end of the century. Many observers felt that scientific understanding was so limited that, as one group concluded in 2007, "the question of whether hurricane intensity is globally trending upwards in a warming climate will likely remain a point of debate in the foreseeable future." However, thorough studies published in 2010 and 2013 concluded that Katrina-like events would become increasingly likely. A 2016 review concluded that while the total number of tropical cyclones might actually decrease, especially intense ones (categories 4 and 5) would come more frequently.

As for storms outside the tropics, computer models differed on whether they would get stronger, but the models agreed that the storm tracks
would shift to different regions. That would probably bring floods more intense
than anyone was prepared to withstand. In addition, ambiguous geological evidence raised speculation that the new climate might include "superstorms" far greater than anything seen in historical times. The very uncertainty of the
matter was a call to action. If there was a serious risk of increased devastation, it was not something we should leave for the
next generation to worry about.(34*)

The sea-level rise
alone makes it likely that low-lying areas where tens of millions
of people live will become uninhabitable by the end of this century.
Entire island nations are at risk. Then it will get worse. Even
if humanity could bring its greenhouse emissions to a full stop, the gases already in the air will capture heat energy
that will work its way gradually deeper into the oceans. The tides
will continue to creep higher, century after century. Meanwhile,
if the planet warms up a few degrees (which is the most likely scenario
unless strong restrictions on emissions are promptly introduced),
the forces melting polar ice will become irreversible. Eventually,
probably after several thousand years, the Greenland Ice Sheet will
be gone. In previous geological ages when the CO2 level in the Earth's atmosphere had reached 500 ppm (a level human emissions would bring within decades), much of Antarctica's ice cap too had been gone, raising the sea level tens of meters. Even if nothing happens in Antarctica, the melting of Greenland will put the
sea level at least seven meters higher, giving posterity its grandest, but
unwelcome, monument of our civilization.(35)

2. Glaciers as "sensitive indicators of climate" are stressed in the
pioneering theoretical treatment of surges, Nye (1960). BACK

3. "The chief conclusion of this paper is that the greatest glacial
fluctuations in Antarctica were produced by changes in sea-level." The paper was motivated by
the idea that the timing of Antarctic glacial movements was set by sea-level changes that
reflected Northern Hemisphere glaciation. Hollin (1962), p. 174.
BACK

4. Wilson (1964); Wilson (1966); Wilson (1969);
Wilson's starting-point was the suggestion that the center of Antarctica was at the pressure
melting point, see Robin (1962), p. 141, who adds that "one
would not expect the ice to surge over a large part of Antarctica at one time"; the role of
frictional heat in ice-sheet instability was pointed out back in 1961 (in partial support of
Ewing-Donn theory), drawing on earlier work by G. Bodvarsson, by Weertman (1961). BACK

27. NOTE that the papers I cite throughout this section
are only examples of numerous papers by these and some other authors.
A readable summary is in Broecker and Kunzig (2008),
pp. 152-56. Wedging: Doake et al. (1998). Ice
base melting: Rignot and Thomas (2002),
"most important finding," p. 1505. Subsequent work pointing
in the same direction included De Angelis and Skvarca
(2003), who found that Antarctic grounded ice surged after an ice
shelf breakup, and Bindschadler et al.(2003),
who reported that a major West Antarctic ice stream started and stopped
flowing as the tide went up and down. Breakup of an ice shelf (Larsen)
leads to a speedup of glacier movement: Rignot
et al. (2004), Scambos et al.(2004) (who
also note lubrication by percolating water, see following note). WAIS
models: Payne (2004); observed WAIS changes:
Thomas (2004), Siegert
(2004). Several papers by Rignot and colleagues document other Antarctic
changes. "A lot shorter:" Robert Bindschadler in Larry Rohter,
"Antarctica, Warming, Looks More Vulnerable," New York Times,
25 Jan. 2005, section D. See Holmes (2004)
for discussion. BACK

32a. Rapid sea-level
changes (10 meters within 1000 years) were found in ancient coral reefs: Thompson and Goldstein (2005); Blanchon et al. (2009) found a "2–3-m jump in sea level" in a century, presumably due to ice sheet instability, during a period warmer than the 20th century. "A rise
of over 1 m by 2100 for strong warming scenarios cannot be ruled out,"
Rahmstorf (2007), extended by Vermeer and Rahmstorf (2009), who project sea-level rise from 1990 to 2100 in the range 75-190cm. The problem will be compounded
in many river deltas (Nile, Ganges, Mississippi, etc.) by a half meter
or so of subsidence as dams impound sediment and water is withdrawn from
aquifers. BACK